(1) Field of the Invention
The invention relates to processes for the manufacture of semiconductor devices and more particularly to processes related to wafer cleaning.
(2) Description of previous art
The fabrication of integrated circuits involves the forming of semiconductor devices within the surface of silicon wafers. The integrated circuits are located within discrete units identified as chips or dice. Each chip or die contains the devices and circuits which will constitute a discrete manufactured product. The dice are arranged in a fashion, on the wafer, to provide a maximum number of functional dice of final product. This organization is commonly identified by a wafer map.
During manufacture, the silicon wafers are subjected to a series of processing steps, some of which address multiple wafers at one time and others of which process the wafers one at a time. In order to effectively monitor the production line it is necessary to maintain a reliable system of wafer tracking. To this end laser identification marks are embedded on each wafer at the very beginning of the processing cycle. They contain information for later chip identification such as lot number and job number.
For some product designs, the laser marks are located in the saw kerf adjacent to the product dice. This means of identification allows in-process tracking of wafers by lot and job number. In other designs laser marks are provided on the chips themselves and can later be used to relate in-the-field chip failures to processing history. Such laser marks reflect not only wafer and lot number but can also indicate die location on the wafer.
The process of laser marking involves instantaneous violent local eruption of material from the surface of the wafer creating a Widespread area of debris consisting of particulates and chemical residue which is visible as a diffuse discoloration in the vicinity of the laser marks after initial oxidation. It is imperative that these particulates and chemical residues be thoroughly removed from the wafers before further processing.
A method using NH4OH/H2O2 and HCL/H2O2 solutions, commonly called RCA cleaning, has been used for cleaning particulates and other chemical residues from silicon wafers for many years. A discussion of the RCA technique may be found in Wolf, S. and Tauber, R. N., xe2x80x9cSilicon Processing for the VLSI Eraxe2x80x9d, Vol. 1, Lattice Press, Sunset Beach, CA, (1986),p516ff. The first ammoniacal solution(SC-1), when used in concert with the application of ultrasonic or megasonic agitation, is effective for removing organic residues and particulates while the second acidic solution(SC-2) is effective in removing metallic contaminants. The abbreviations SC-1 and SC-2 are commonly used and refer to standard clean 1 and 2. A dilute HF treatment is often added between SC-1 and SC-2 to remove silicon oxide.
The RCA method may also be used to remove of residues left as a result of laser ablation during the formation of wafer identification marks. A current procedure applies SC-1 in a megasonic bath followed by three quick-dump rinses in un-heated DI (de-ionized) water. This is then followed by a dip in 1%HF followed by a second SC-1 and an SC-2 treatment. The megasonic SC-1 treatment is found to be effective in removing the particulates but chemical residues remain and must be removed by the SC-2 treatment.
Li, U.S. Pat. No. 5,681,397 teaches a method of rinsing and drying silicon wafers after their surfaces have been treated by aqueous HF. The method comprises immersing the wafers in a boiling hydrochloric acid solution and drawing the wafers out to dry in ambient air. Experiment found that the surface roughness is less with boiling HCL than with boiling water alone.
Ludwig, et. al., U.S. Pat. No. 5,000,208 describes an apparatus for rinsing and drying wafers but does not address hot water rinsing. Bhat, U.S. Pat. No. 5,589,422 describes a gas phase process for removing trace metals as well as thin surface layers from a semiconductor wafer. The process forms a thin oxide film which is then removed by gas phase etching. Vetter, U.S. Pat. No. 5,069,235 describes an apparatus for cleaning and rinsing wafers which features the capability to perform quick dumps and spray rinses.
It is an object of this invention to provide an improved method for cleaning silicon wafers.
It is another object of this invention to provide a cost effective method for cleaning wafers of particulate and chemical debris left after forming marks by laser ablation.
It is another object of this invention to provide a method for cleaning wafers of particulate and chemical debris left after forming marks by laser ablation which eliminates the hazardous use HF.
It is another object of this invention to provide an improved method for removing chemical residues left after megasonic cleaning of wafers in ammoniacal/hydrogen peroxide solutions,
It is another object of this invention to reduce chemical consumption and cleaning machine utilization and improve wafer throughput in a manufacturing line.
These objects are accomplished by first subjecting the wafers to a cleaning bath containing a heated RCA SC-1 ammoniacal/peroxide solution under megasonic agitation, followed by a rinse in heated DI water. The temperature of the DI water must be greater than 50xc2x0 C. Thereafter three room temperature (25xc2x0 C.) DI water quick-dump rinses are performed followed by spin drying. The incorporation of the hot DI water rinse eliminates the need for treatment with the second RCA acidic/peroxide solution and thereby affords considerable cost savings in terms of chemical usage and disposal as well as equipment time. The need for HF is eliminated from the process, thereby greatly improving safety.